The present invention relates to a spindle motor to be mounted to, for example, a hard disc drive, an optical disk drive, a magneto-optical disc drive, a magnetic disc drive or a polygon mirror.
Various kinds of spindle motors are available in the market, and thus a key component of these spindle motors, i.e., a fluid bearing device, is of many types. Many of the fluid bearing devices include a cylindrical sleeve, which bears a load of a shaft in a radial direction. The sleeve surrounds the shaft via an annular space between an outer wall of the shaft and an inner wall of the sleeve. This space is called xe2x80x9ca radial bearing spacexe2x80x9d and is filled with lubricant. When a rotor-hub fixedly mounted to a first end of the shaft rotates, hydrodynamic-pressure generating grooves (e.g. herringbone grooves) generate dynamic pressure in the lubricant, so that a radial bearing is formed. The grooves are provided on the outer wall of the shaft and the inner wall of the sleeve.
A disc-shaped thrust plate is fixed to a second end of the shaft, and herringbone grooves, for instance, are provided on at least one of an upper face or a lower face of the thrust plate. Lubricant is filled into these grooves, so that a thrust bearing is formed in order to bear a load in an axial direction.
A spindle motor and bearing device as discussed above is disclosed in Japanese Patent Application Non-examined Publication No. 2000-113582 (hereinafter called xe2x80x9cprior art 1xe2x80x9d). Prior art 1 teaches the following points in order to prevent lubricant from splashing out from bearings: (a) A slope flaring downwardly in an axial direction is provided on an outer wall of a supporting member, namely, the sleeve. (b) A slope is provided at an upper end of the supporting member so that a space of a thrust bearing flares outwardly in a radial direction. This space is called a thrust bearing space. (c) Oil repellent made of fluorine-based material is applied to a tapering-sealed section in order to prevent oil-migration, i.e., lubricant such as oil, from leaking out from the bearings.
Japanese Patent Application Non-examined Publication No. 2000-121986 (prior art 2) discloses an idea that lubricant in a radial bearing space is steadily retained, which prevents the lubricant from splashing out or leaking out of the bearing space. In other words, an annular recess is provided on an inner wall of a sleeve, and the recess has a slope flaring toward an opening of the sleeve. FIG. 6 of prior art 2 illustrates that a conical section 103c is provided at an opening end of sleeve 103, and conical section 103c has a slope recessed from an outer wall to the inner wall. When shaft 102 is inserted into sleeve 103 in assembling the motor, even if lubricant overflows from a bearing space, the overflowed lubricant flows back to the space along this conical section 103c provided at an end face of sleeve 103. This structure prevents the lubricant from splashing out from the bearing space.
Both of prior art 1 and prior art 2 disclose an idea of preventing lubricant from leaking out. Prior art 1 discloses that the slope is provided on an outer wall of the sleeve, and the slope flares downwardly in the axial direction in order to prevent lubricant from splashing out. Indeed, this structure prevents lubricant, retained in the thrust bearing space, from splashing out; however, prior art 1 does not disclose anything about problems and their solutions with regard to filling lubricant into a thrust bearing space.
Prior art 2 raises a problem that lubricant overflows when a shaft is inserted into a sleeve while assembling a spindle motor, and proposes a solution in that a slope is provided at an end face of the sleeve. However, according to prior art 2, surplus lubricant is preferably prepared before assembly because prior art 2 refers to a method of inserting a shaft into a sleeve in which lubricant is filled.
The present invention aims to provide a spindle motor that comprises the following elements:
(a) a shaft;
(b) a flange fixedly mounted to a first end of the shaft and having a diameter greater than that of the shaft;
(c) a cylindrical sleeve to be fit to the shaft;
(d) a radial bearing space is to be formed between an outer wall of the shaft and an inner wall of the sleeve, which is opposite and close to the outer wall of the shaft;
(e) a rotor hub fixed to a second end of the shaft and facing an upper end of the sleeve;
(f) a thrust plate disposed close to and opposite the flange;
(g) a thrust bearing space formed between faces of the flange and the thrust plate which are close to and opposite each other;
(h) lubricant retained in the thrust bearing space; and
(i) a tapering section provided at an upper end of the sleeve.
This structure allows a thrust bearing to bear an axial load of the shaft, because the flange has a diameter greater than that of the shaft, and the thrust bearing is disposed between the faces of the flange and the thrust plate which are close to each other. Further, because the tapering section is provided at the upper end of the sleeve, a sufficient amount of lubricant to be filled into the radial bearing space can be stored. The construction discussed above restrains the lubricant from splashing or leaking out from the sleeve, and lowers pressure applied to the lubricant so that bubbles do not occur in the lubricant. As a result, a spindle motor having a longer service life is obtainable.